1. Field of the Invention
The present invention relates to an improvement in a process for removing particulate and aerosol droplets from a stream of gases. More specifically, the present invention relates to a more effective system for removing entrained particles and droplets of tar from a gas stream originating from a source such as a biomass gasifier so that the resulting cleaned gas stream is suitable fuel for operating an internal combustion device, such as an engine or turbine, which may be coupled to an electrical generator or can be utilized as a synthetic gas for subsequent processing. For the purposes of simplicity, an internal combustion device is discussed herein.
2. Description of the Related Art
Developing countries need decentralized sources of power, i.e. power systems for each remote community. In developing countries, where natural gas, petroleum products, or coal are not readily available to remote communities and hydropower is not possible, communities often have some local form of biomass that could serve as an energy source if that biomass could be converted to electrical power. Locally available forms of biomass might include rice straw or rice hulls, sugar cane bagasse, poultry litter, refuse, paper plant pulp sludge, switchgrass, waste resulting from extraction of olive oil from olives, peanut shells, sawdust or wood chips, wood bark, municipal solid waste, coconut shells, corn cobs, cotton stover, etc.
Industrialized nations have a heightened awareness of the environmentally deleterious effects of the production of xe2x80x9cgreenhouse gasesxe2x80x9d including carbon dioxide produced by the combustion of fossil fuels. Many nations have agreed to aggressively reduce their production of these xe2x80x9cgreenhouse gasesxe2x80x9d by encouraging the use of alternate, renewable energy such as biomass. A concurrence of nations was reached during the summit conference on the environment that was held in Kyoto, Japan several years ago.
Technology is currently available for converting biomass materials, by heating the biomass materials under starved oxygen conditions, to a gas stream that has sufficient heating value to operate an internal combustion device, i.e. in the range of 125 to 250 BTUs per standard cubic foot, depending on the biomass materials being processed. The resulting gas stream contains nitrogen, carbon dioxide, trace amounts of carry-over ash and tar, and calorific constituents of carbon monoxide, hydrogen, and some alkanes and alkenes. Gasification is recognized worldwide as an innovative method of converting biomass into energy.
However, one of the problems that has been experienced with converting biomass to energy is that the gas stream that is produced by gasification units is contaminated with particulate matter and with aerosol droplets of tar that can foul an internal combustion device unless they are efficiently removed from the gas stream prior to introducing the gas stream into the device. Currently there is not an economical method for effectively removing the entrained particulate matter and the aerosol droplets of tar from these types of gas streams. The reason that the particulate matter and aerosol droplets of tar can not be easily be removed from the gas stream is that a large portion of the particles and droplets are micron to sub-micron in size and are not effectively removed by traditional gas scrubbing processes.
The invention addresses this problem by first passing the gas that is generated by the gasifier through a high temperature cyclone separator to remove most of the carry over ash from the gasifier to prevent fouling of the downstream equipment. Then the gas that exits the cyclone separator is passed through an indirect gas cooler, a direct contact spray scrubber chamber, then followed by one or more enhanced vortex chambers. To achieve the desired cleanliness in the resulting gas stream, it may be necessary to employ two or more vortex chambers in series. When the gas exits the vortex chambers, it passes through an inducted draft fan, then finally through a heat exchanger where the gas is cooled to condense additional impurities and then a demister or mist eliminator where those impurities are extracted from the gas.
The high temperature cyclone separator operates at approximately 1,000 degrees Fahrenheit, thus removing the entrained fly ash without cooling appreciably. The cyclone separator is a conventional type of cyclone separator designed for high temperature operation. The cyclone separator removes approximately 90% of the ash that is carried over in the gas from the gasifier. Removal of this ash prevents fouling of the downstream equipment, particularly the indirect gas cooler located immediately downstream of the cyclone separator. Also, removal of the ash reduces the loading on the direct contact spray scrubber located downstream of the indirect gas cooler. Thus, the addition of a high temperature cyclone separator between the gasifier and the indirect gas cooler allows the system to operate for longer periods of time without being brought down for cleaning and allows the system to do a better job of cleaning the gas.
The indirect gas cooler is a shell and tube heat exchanger that cools the gas stream from the gasifier by indirect heat exchange with a cooling medium such as air or water. The direct contact spray scrubber employs a liquid hydrocarbon, such as used motor oil, to scrub out the particulate matter and some of the organic aerosols that are entrained in the gas stream as the gas stream passes through the direct contact spray scrubber.
Once the gas exits the direct contact spray scrubber, it enters the enhanced vortex or vortices. Each enhanced vortex chamber employs a high-speed fan to propel the remaining entrained droplets of tar against the inside surface of the vortex chamber along with additional oil. When the droplets of tar hit the oil coated inside surface of each vortex chamber, the droplets coalesce on the surface. The tar and oil mixture then gravity flows out of each vortex chamber, thereby removing the tar from the gas stream. The gas stream, having thus been cleaned of its particulate and aerosol impurities, then enters a low-pressure surge tank. If the gasifier is operating at a pressure less than atmospheric pressure, an induced draft fan may be employed to convey the gas through the system.
When the gas exits the vortex chambers, it passes through a heat exchanger that cools the gas to less than 120 degrees Fahrenheit, thus condensing additional impurities and water. Finally, the gas passes through a demister where the condensed impurities and water are extracted from the gas.
From here, the gas stream can be sent directly to the internal combustion device for mixing with combustion air so that it can be burned in such internal combustion device, such as an engine or turbine, which may be coupled to an electrical generator or can be utilized as a synthetic gas for subsequent processing.
The present invention is an improvement in a method for removing particulate matter and aerosols from a gas stream generated by a biomass gasification unit. The invention consists of first removing the excess ash by passing the gas through a high temperature cyclone separator, then cooling the gas stream, oil scrubbing it to remove particulate matter and some tars and to further reduce the temperature of the gas stream, passing the gas stream through one or more vortex chambers to remove additional tars, and finally cooling the gas to condense out more impurities and water and removing the condensate with a demister.
The high temperature cyclone separator operates at approximately 1,000 degrees Fahrenheit, thus removing the entrained fly ash without cooling the gas appreciably. The cyclone separator is a conventional cyclone separator designed for high temperature operation. The cyclone separator removes approximately 90% of the ash that is carried over in the gas from the gasifier. Removal of this ash prevents fouling of the indirect gas cooler located immediately downstream of the cyclone separator, and reduces the particulate loading on the direct contact spray scrubber located downstream of the indirect gas cooler. Thus, the removal of the majority of the ash by the cyclone separator allows the system to operate for longer periods of time without being brought down for cleaning.
Once the gas stream exits the cyclone separator, it passes through an indirect gas cooler to reduce the temperature of the gas stream to a temperature that will not crack petroleum scrubbing liquor, i.e. a temperature below approximately 600 degrees Fahrenheit. If the gas stream is cooled below 450 degrees Fahrenheit, tars may condense in the indirect gas cooler, thereby restricting gas flow. Therefore, the most desirable temperature range is between 450 and 600 degrees Fahrenheit. Cooling of the gas stream is necessary since the gas exits the gasification unit at a high temperature, i.e. approximately 1200-1500 degrees Fahrenheit. The gas stream must be cooled to a temperature that will not crack petroleum products, such as the motor oil, since the gas stream will come in contact with the petroleum scrubbing liquor when it enters the next vessel in the process, i.e. a direct contact spray scrubber. The indirect gas cooler employs indirect heat exchange with air or water to cool the gas stream to an acceptable temperature. To minimize gas flow restriction from impurity accumulation, the minimum heat exchanger tubing size should not be less than 2 inch.
Upon leaving the indirect gas cooler, the gas stream enters a direct contact spray scrubber. The direct contact spray scrubber employs a liquid hydrocarbon, such as used motor oil, to directly scrub and cool the gas stream and remove the particulate matter, some of the organic aerosols, and some water that is entrained in the gas stream. Within the direct contact spray scrubber, a petroleum product or oil, such as used motor oil, is sprayed into the gas stream countercurrent to the direction of flow of the gas stream to scrub out particulate matter and some of the tar droplets contained in the gas stream. Some of the excess water also is removed by condensation in the direct contact spray scrubber since the temperature of the gas stream falls below the water vapor dew point within the direct contact spray scrubber. This water condensation occurs around sub-micron particle seed that promotes particle growth. The enlarged particles are more effectively removed in the downstream, enhanced vortex chamber or chambers. The gas stream exits the direct contact spray scrubber at a temperature of approximately 100 to 150 degrees Fahrenheit.
Upon exiting the direct contact spray scrubber, the gas stream enters an enhanced vortex chamber or chambers, if more than one vortex chamber is employed. Here the gas stream is mechanically scrubbed of tar. Additional motor oil is sprayed into the gas stream as the gas stream enters each vortex chamber. The oil and gas stream mixture enter each vortex chamber adjacent to or beneath a high-speed fan that propels the gas stream, oil, and entrained droplets of tar against the inside surface of the vortex chamber. It is believed that the vortex created by the rapidly rotating fan forms a low-pressure zone. Within this low-pressure zone, tars with a partial vapor pressure in excess of the zone pressure condense. It is further believed that the maximum fan tip speed for the high-speed fan is approximately 300 M.P.H. since fan tip speeds in excess of this speed may result in metal fatigue of the fan blades.
When the droplets of tar contact the inside surface of the vortex chamber, they adhere to the oil-coated surface and coalesce with the oil on the surface. The oil also impinges on the fan blades and serves to keep the blades cleaned of tar that might otherwise accumulate. The temperature of the gas stream is approximately 100 to 150 degrees Fahrenheit when it initially enters the first vortex chamber and is approximately 125 degrees Fahrenheit when it exits the last vortex chamber. The increase in temperature of the gas stream as it passes through the vortex chamber or chambers is due to the heat of compression.
Because the temperature of the interior of each vortex chamber is above the pour point temperature, the tar will flow down the wall of each vortex chamber into a sump and can be disposed of via a tar pump that connects to the sump of each vortex chamber.
When the gas exits the vortex chambers, it passes through a heat exchanger that cools the gas to less than 120 degrees Fahrenheit, thus condensing additional impurities and water. The heat exchanger may employ water or other suitable coolant as a cooling medium. Upon exiting the heat exchanger, the gas finally passes through a demister where the condensed impurities and water are extracted from the gas.
Upon leaving the demister, the gas stream is sufficiently free of particulate matter and tar that it can be burned in an internal combustion device without fouling or can be subsequently processed. Any residual particulate matter or hydrocarbon aerosols contained in the gas stream at this point are sub-micron in size and are not sufficient to cause any deterioration in extended operation of down-stream equipment.
Prior to introduction of the cleaned gas stream into down-stream equipment, the gas stream will enter a low-pressure surge tank. If the gasifier is operating at less than atmospheric pressure, an induced draft fan conveys the gas through the system and into the surge tank. The induced draft fan is located upstream of the heat exchanger and demister. The surge tank is sized in residence time to provide gas mixing, compensating for fluctuations in the gasification process. Also the surge tank serves as a final knock out drum for removal of any remaining liquids from the gas stream. The temperature of the gas stream as it enters the surge tank is less than 120 degrees Fahrenheit. From the surge tank, the gas stream flows to an internal combustion device or to subsequent processing.